I'm going to assume that when you write "design", you actually intend "create", so that your question is more properly read, "How can I learn to create content?".
But "content" is a somewhat nebulous term that incorporates just about everything in Trainz. The collection of baseboards, with or without terrain modifications, is content, and this can be created by using the terrain modification tools in surveyor. Planning activities involving the operation of trains and engines, and arranging the activities in an order, the order called a "session", is also content, and involves a different set of skills, also performed using surveyor. Creating assets--railroad related (cars, locomotives, buildings and bridges, track) and non-railroad (houses, businesses, manufacturing plants, warehouses, churches, schools, houses, &c.) involves the use of 3d modeling software, and 2d image processing software external to Trainz.
Your post implies the last category, creating assets. You say "you're going to use" Adobe Illustrator / Photoshop, but you don't indicate your level of proficiency, and you don't mention 3d software at all. If you're going to create content, whether rail or non-rail, the first step is to have a good working knowledge of the software. If you don't already have the image processor, and want to preserve your financial assets, I suggest you investigate the image processing software GIMP. It is available for free download, and is open source, and will be more than adequate to do image processing duties, but it, like photoshop, are sophisticated programs which will require some time and effort to learn to use to best effect.
Same is true for 3d modeling. There are a number of packages available for use, at varying pricepoints, from the Open Source Blender, GMAX, and Sketchup, to pricier products like 3dCrafter to even more highly priced products like 3DStudio. You'll need to become facile with one of these packages if you wish to create meshes. Once you know how to use the software, it's not hard at all.
My personal preference is Blender and GIMP, because I am a strong proponent of Open Source software. I was using GIMP before I ever started in Trainz, and saw no reason to learn anything else. I originally started with GMAX as a 3d modeling tool, but when I learned that GMAX is no longer being supported, I switched to Blender.
Once you know how to use the tools, the rest is easy. There are specific things that have to exist in the models, but by the point you get to needing to know about these things (and there are tutorials available to help learn these things), they will be trivial to do.
ns
